Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament B. Forestier, A. Houard, I. Revel, M. Durand, Y. B. André et al. Citation: AIP Advances 2, 012151 (2012); doi: 10.1063/1.3690961 View online: http://dx.doi.org/10.1063/1.3690961 View Table of Contents: http://aipadvances.aip.org/resource/1/AAIDBI/v2/i1 Published by the American Institute of Physics. Related Articles Arc-based smoothing of ion beam intensity on targets Phys. Plasmas 19, 063111 (2012) Tesla coil discharges guided by femtosecond laser filaments in air Appl. Phys. Lett. 100, 181112 (2012) Stability of very-high pressure arc discharges against perturbations of the electron temperature J. Appl. Phys. 111, 073305 (2012) Modeling of switching delay in gas-insulated trigatron spark gaps J. Appl. Phys. 111, 053306 (2012) Three-dimensional model and simulation of vacuum arcs under axial magnetic fields Phys. Plasmas 19, 013507 (2012) Additional information on AIP Advances Journal Homepage: http://aipadvances.aip.org Journal Information: http://aipadvances.aip.org/about/journal Top downloads: http://aipadvances.aip.org/most_downloaded Information for Authors: http://aipadvances.aip.org/authors Downloaded 08 Jul 2012 to 69.254.253.11. All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ AIP ADVANCES 2, 012151 (2012) Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament B. Forestier,1 A. Houard,1 I. Revel,2 M. Durand,1 Y. B. Andre,´ 1 B. Prade,1 A. Jarnac,1 J. Carbonnel,1 M. Le Neve,´ 3 J. C. de Miscault,3 B. Esmiller,4 D. Chapuis,3 and A. Mysyrowicz1 1Laboratoire d’Optique Appliquee,´ ENSTA ParisTech, Ecole Polytechnique, CNRS, Palaiseau, 91761, France 2EADS France, Innovation Works, France 3CILAS, Laser sources development and Industrialization, Orleans,´ France 4ASTRIUM, Space Transportation, Les Mureaux, France (Received 17 October 2011; accepted 7 February 2012; published online 17 February 2012) In the perspective of the laser lightning rod, the ability of femtosecond filaments to trigger and to guide large scale discharges has been studied for several years. The present paper reports recent experimental results showing for the first time that filaments are able not only to trigger and guide but also to divert an electric discharge from its normal path. Laser filaments are also able to divert the spark without contact between laser and electrodes at large distance from the laser. A comparison between negative and positive discharge polarities also reveals impor- tant discrepancies in the guiding mechanism. Copyright 2012 Author(s). This ar- ticle is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.3690961] I. INTRODUCTION If lightning is one of the most fascinating phenomena occurring in the atmosphere, it is also one of the most dangerous. This spark discharge of several kilometers can cause severe damages to ground infrastructures. During history, several techniques have been developed for lightning protection, such as Benjamin Franklin’s famous lightning rod or the rocket triggering device.1 The laser lightning rod would be a valuable alternative to lightning rockets. This concept relies on the generation by powerful lasers of a long plasma column acting like an extension of the classical rod toward thunder clouds and would be able to significantly empty electrically charged clouds preventing lightning stroke to hit sensitive building or facilities. Imagined in the early 60’s the concept of laser-triggered discharges was first investigated with 2 3 high energy CO2 and YAG lasers. Despite the first real scale demonstration of triggering in 1996, this path was progressively abandoned because of the discontinuous profile of the plasma generated with such long pulses through avalanche breakdown.4 Following the development of femtosecond CPA (chirped pulse amplification) laser systems, the study of ultrashort filaments in air5 and their ability to generate a thin uniform plasma channel over very long distances opened new perspectives in the field.6–10 Laser filamentation is a nonlinear propagation regime affecting femtosecond laser beams pro- vided their peak power exceeds a critical value (5 GW at 800 nm in air). A dynamical competition between Kerr effect which tends to self focus the beam, diffraction and multiphoton ionization which defocus the beam leads to the formation of self guided light pulses called filaments maintaining a high peak intensity over long distances.11–13 These self guided pulses leave in their wake a uniform weakly ionized plasma column.14, 15 Laboratory scale experiments of large discharges guiding by filamentation in a plane rod geometry have demonstrated the ability of filaments to decrease the breakdown threshold by 30%7, 8 and to guide the spark over 2 to 3 meters for positive upward leader9–11 and for negative upward leader.18 2158-3226/2012/2(1)/012151/13 2, 012151-1 C Author(s) 2012 Downloaded 08 Jul 2012 to 69.254.253.11. All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ 012151-2 Forestier et al. AIP Advances 2, 012151 (2012) Planar electrode HV V 2.5 m Spherical electrode 11 m Lens Laser I1 FIG. 1. Experimental setup. In this manuscript we study the guiding efficiency of filaments with a positive and negative voltage in plane/rod electrode rod geometry. We also report the first demonstration of the possibility to deviate a long spark discharge from its natural point of attachment. Finally the guiding of discharge without contact between laser and electrodes is investigated. II. INFLUENCE OF THE VOLTAGE POLARITY A. Experimental setup Experiments were performed at the DGATA center in Toulouse in the high voltage facility FOUDRE. The first experimental setup is presented in Figure 1. A large planar electrode connected to a high voltage Marx generator was placed 2.5 m above a spherical one (15 cm of diameter) connected to the ground. The high voltage generator could deliver up to 2 MV in both polarities. The voltage applied consisted in a standard voltage waveform modeling a fast lightning process. To produce the plasma filament, the laser ENSTAmobile built by Amplitude Technologies was used. This laser is a mobile Ti:Sa CPA laser chain delivering pulses of up to 350 mJ energy with a duration of 50 fs (7 TW) at a repetition rate of 10 Hz. To postpone the onset of filamentation and transport the beam without damaging the optics, a linear chirp of about 15000 fs2 was impressed to the pulse. The laser beam of 40 mm diameter was weakly focused in order to create the plasma filaments tangentially to one side of the sphere electrode. The plasma column was composed of a bundle of 80 filaments starting at a distance of 7 m after the lens and continuing over a distance of 4 m. In the gap separating the two electrodes, the multiple filaments formed a quasi homogeneous circular plasma column several mm in diameter. The current I1 circulating through the electrode was measured with a Rogowski coil, while the voltage V on the planar electrode was measured through a resistive probe. B. Results and discussion Figure 2(a) shows a still image of an unguided discharge obtained in the absence of laser with a voltage V = -1.3 MV applied to the plane electrode. The corresponding temporal evolution of the Downloaded 08 Jul 2012 to 69.254.253.11. All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ 012151-3 Forestier et al. AIP Advances 2, 012151 (2012) (a) (b) 0.0 0 C u rrent I -0.5 -2 1 (kA) -1.0 Voltage V ( MV) -4 -1.5 01020 Time (μs) (c) (d) 0.0 0 C u rrent I -0.5 -2 1 (kA) -1.0 Voltage (MV) Voltage τ L -4 -1.5 01020 Time (μs) FIG. 2. Integrated picture of the discharge and measurements of the voltage and current in the case of an unguided discharge (a,b) and laser guided discharge (c,d). voltage on the charged electrode and the current circulating in the spherical electrode is presented in Figure 2(b). Figures 2(c) and 2(d) show the same measurement when a plasma filament is formed between the electrodes at time t = 0 s, during the voltage rising front. In this case the discharge path is perfectly straight and follows the filament axis, showing that the guiding is achieved over the full length of the discharge. It has been shown that the plasma column formed by laser filaments can lower the breakdown fieldby10to30%.8, 9 Here we investigate the evolution of this effect as a function of the delay τ L between the beginning of the voltage front and the laser filament formation. The average peak electric field between the electrodes is defined as the maximum applied voltage divided by the separation gap. The value of this average field is plotted in Figure 3 as a function of τ L for a positive (Figure 3(a)) and a negative applied voltage (Figure 3(b)). The natural breakdown field in absence of laser filament is measured for both polarities and indicated in the graphs by the dashed orange line. It is equal to 8 kV/cm for a positive voltage polarity and to 5.2 kV/cm for negative voltage polarity, which are close to the stability field amplitudes for streamer reported by Gallimberti et al..19 As shown by the blue stars in Figure 3, when the discharge is guided over the full length by the filament, the breakdown field can be decreased to 3.6 kV/cm for positive voltage and to 4.3 kV/cm for negative voltage.